Question: $ B = \left[\begin{array}{rrr}1 & 2 & 1 \\ 2 & 0 & 1 \\ 0 & 1 & 1\end{array}\right]$ What is $ B^{-1}$ ?
Answer: $ = \left[\begin{array}{rrr}\frac{1}{3} & \frac{1}{3} & -\frac{2}{3} \\ \frac{2}{3} & -\frac{1}{3} & -\frac{1}{3} \\ -\frac{2}{3} & \frac{1}{3} & \frac{4}{3}\end{array}\right]$